The present invention generally relates to communication signal processing and particularly relates to measuring received signal quality.
Many types of communication systems employ some form of transmit power control, wherein the transmit power of a transmitter is controlled explicitly or implicitly in response to feedback from a remote receiver. For example, interference limited systems, such as Code Division Multiple Access (CDMA) wireless communication networks, use forward and reverse link power control to control interference and promote efficient use of available transmit power resources.
With respect to a given mobile terminal, forward link power control comprises receiving power control commands from the mobile terminal at a supporting network base station and adjusting the forward link transmit power allocated for serving that mobile terminal up and down according to the incoming commands. If the received forward link signal quality at the mobile terminal is below a targeted value, the mobile terminal sends one or more up commands and, conversely, if the quality is above the target, it sends one more down commands. Such comparisons and the corresponding command generation typically occurs at a relatively high rate, e.g., hundreds of times per second, such that the transmitting base station(s) continually adjust the forward link power for the mobile station to maintain targeted signal quality. This closed-loop mode of power control is referred to as “inner-loop” power control.
“Outer-loop” complements the above inner loop operations by adding a second but typically much slower adjustment mechanism that adjusts the inner-loop control target based on some other received signal metric. For example, the mobile station may evaluate the Frame Error Rate (FER), or evaluate some equivalent metric, of a received forward link traffic channel as the basis for outer loop control. Thus, if the FER is too high with the current received signal quality target, the outer-loop control mechanism adjusts the inner loop target upward, by 1 dB for example. Conversely, if the FER is below the acceptable error rate, the outer-loop control mechanism might adjust the inner-loop target downward.
With the above inner-loop and outer-loop control mechanisms, the receiving mobile station ensures that the network transmitters dynamically adjust their transmit powers as needed to maintain targeted signal quality at the mobile terminal over changing radio conditions, and ensures at the same time that the transmitters do not transmit at more power than is needed. Similar inner-loop and outer-loop power control may be implemented at the network base stations for the reverse links between those base stations and respective ones of the mobile terminals being supported by them. As with the forward link, reverse link power control ensures that each mobile station transmits with no more power than is needed to maintain targeted received signal quality (and data error rates) at the supporting base stations.
While the above link power control is straightforward in description, in practice such power control can be complicated by the requirement to timely and accurately determine the received signal quality at the inner-loop power control rate. For example, Wideband CDMA (WCDMA) specifications require mobile terminals to compute received signal quality and return corresponding Transmit Power Control (TPC) commands (bits) relatively early in each transmit slot. Because signal quality in such environments depends on the characteristics of multipath signal reception from one or more network transmitters, determination of actual received signal quality can be computationally complex.